1. Field of the Invention
The present invention relates to a material of lead frames for semiconductor devices, having strength greater than those of known materials. The invention also is concerned with a method of producing this material.
2. Description of the Related Art
In recent years, semiconductor devices such as logics etc. are required to have greater capacities, higher degree of integration and thinner wall thickness regarding a package. Under this circumstance, there is a trend for a greater number of pins to be installed on lead frames of such semiconductor devices and for smaller wall thickness of the lead frames. This has given a rise to the demand for a lead frame material having a greater strength.
Hitherto, materials such as Fe-42Ni and Fe-29Ni-17Co (kovar) are known as materials for multi-pin ferrous-type lead frame. Various proposals have been made for improving properties of these known materials. For instance, Japanese Unexamined Patent Publication No. 55-131155 discloses a high-strength Fe-Ni type alloy containing various strengthening elements. Improved alloys of Fe-Ni-Co type are disclosed in Japanese unexamined Patent Publication Nos. 55-128565. 57-82455 and 61-6251, Japanese Examined Patent Publication Nos. 1-817 and 1-15562, and Japanese Unexamined Patent Publication No. 1-61042 of the same applicant as the present invention.
In most cases, a multi-pin lead frame is produced by photo-etching which is suitable for fine and delicate processing. Thin-walled multi-pin lead frames of Fe-42Ni or Fe-29Ni-17Co, produced through a delicate processing, tend to suffer from various problems. For instance, they are often apt to exhibit deformation such as warping, bending and so on during assembling of package, transportation and mounting, due to insufficient strength of the lead, as well as buckling due to shock during the use.
As mentioned before, various attempts and proposals have been made for improving Fe-Ni type alloys and Fe-Ni-Co type alloys. Namely, Japanese Unexamined Patent Publication No. 55-131155 proposes to improve strengths in Fe-Ni type alloys and Fe-Ni-Co type alloys by addition of Si, Mn and Cr. There are also various proposals for strengthening these alloys by addition of strengthening elements other than those mentioned above. It has also been proposed to control thermal expansion of Fe-Ni-Co type alloys as in, for example, Japanese Unexamined Patent Nos. 55-128565, 57-82455 and 61-6251, Japanese Examined Patent Publication Nos. 1-817 and 1-15562 and Japanese Unexamined Patent Publication No. 1-61042. The alloys strengthened by addition of strengthening elements, however, are liable to oxidize the surface of the lead frame, due to excessive amounts of strengthening elements besides the major elements, resulting in serious degradation in solderability and platability both of which are very essential factors for lead frames. Among the above-mentioned Patent Publications proposing control of thermal expansion of Fe-Ni-Co type alloys, only Japanese Unexamined Patent No. 55-128565 attempts to positively improve the strength of the lead frame. The art disclosed in this patent publication, however, employs a strengthening mechanism which is distinct from that of the material of the present invention.